Core: Make member functions const/static

readability-make-member-function-const finds lots of member functions
that could be made const. This change just picks getter functions that
really should be const.

readability-convert-member-functions-to-static finds non-static member
functions which do not access this. This change turns most of them
into static ones, but leaves some non static to keep the class API
consistent.

Change-Id: I004d6f7fab4375f9902f940f29b4a4f561fc7604
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/coreplugin.h

@@ -87,7 +87,7 @@ private slots:
 
 private:
     static void addToPathChooserContextMenu(Utils::PathChooser *pathChooser, QMenu *menu);
-    void setupSystemEnvironment();
+    static void setupSystemEnvironment();
     void checkSettings();
 
     MainWindow *m_mainWindow = nullptr;